The present invention relates to a novel large pore multi-dimensional phosphorous-containing zeolite with structural type CON, its preparation and its use, in particular in a process for catalytic cracking of a hydrocarbon feed. More precisely, it relates to a borosilicate molecular sieve which can be transformed into an aluminosilicate and stabilized by at least one phosphorous-containing compound.
Zeolitic molecular sieves are crystalline materials comprising a network of three-dimensional TO.sub.4 tetrahedra where T is Si, Al, B, P, Ge, Ti, Ga, Fe, for example. This network defines a microporous intracrystalline network with dimensions which are comparable to those of small to medium organic molecules. The microporous network can be a system of channels and/or cavities, shaped by the crystalline network, and which can be identified by its particular and specific X ray diffraction diagram.
The potential applications of a zeolite (for example in catalysis, adsorption, cation exchange and purification processes) depend principally on the size, shape and characteristics (uni-, multi-dimensional) of their microporous network and their chemical composition. As an example, in aluminosilicate type zeolites, the presence of AlO.sub.4.sup.- tetrahedra isolated in a matrix of SiO.sub.4 tetrahedra requires the presence of compensation cations to counter-balance the negative charge of the framework. These cations are typically highly mobile and can be exchanged with others, for example H.sup.+ or NH.sub.4.sup.+, the latter being able to be transformed into H.sup.+ by calcining, resulting in an acidic microporous solid. The zeolite is then in its acid form also known as its hydrogen form. When all of the compensating cations are organic alkylammonium or ammonium cations, calcining leads directly to the acid form of the zeolite. Such microporous acidic solids can be used in acid catalysis processes and their activity and selectivity depend on the strength of the acid, on the size and the on dimensional characteristics of the space delimited in the framework, in which the acidic sites are found.
The channel size can be described by the number of TO.sub.4 tetrahedra present in the ring delimiting the pore openings, which element controls the diffusion of molecules. Thus the channels are classified into categories: small pore openings (annular pore openings delimited by a sequence 8 TO.sub.4 tetrahedra (8 MR, medium pores (10 MR) and large pores (12 MR), MR standing for membered ring.
The prior art is illustrated in U.S. Pat. No. 5,110,776, International patent application WO-A-9507859 and the publication by S. I. Zones et al., "Boron-beta zeolite hydrothermal conversions: The influence of template structure and boron concentration and source", Microporous Material, Elsevier, vol. 2, 1994, Amsterdam, NL, pages 543-555.
Of the numerous zeolites which have already been described in the literature, zeolites with structural type CON, i.e., SSZ26 zeolite (PCT/US 89/01179, 1989), SSZ-33 zeolite U.S. Pat. No. 4,963,337) and CIT-1 zeolite (J. Am. Chem. Soc., 1995, 117, 3766) are of interest as they are the only synthetic zeolites known to possess an interconnected medium (10 MR) and large (12MR) pore system.
This structural characteristic can result in interesting form selectivity properties in such materials for heterogeneous catalysis. The term "form selectivity" is generally used to explain specific catalytic selectivities due to steric constraints which exists inside the microporous zeolitic system. Such constraints can affect the reactants (diffusion of reactants in the zeolite), the reaction products (formation and diffusion of the formed products from the zeolite), the reaction intermediates or the reactional transition states which form in the micropores of the zeolite during the reactions. The presence of appropriate steric constraints can in some cases prevent the formation of reaction intermediates and transition states leading to the formation of unwanted products and can in some cases improve selectivities.
The family of 10/12 MR zeolites consists of intergrowths of two polymorphs in different proportions. The differences between the SSZ-26, SSZ-33 and CIT-1 zeolites originate from the preparation method, chemical composition (zeolites/as synthesized) and the relative proportion of polymorphs in the intergrowth. Thus CIT-1 and SSZ-33 zeolites each belong to the category of as synthesized borosilicate materials. The term "as synthesised" means any zeolite obtained directly from its preparation with no intermediate modification. CIT-1 zeolite is substantially constituted by a single polymorph, while SSZ-33 is an intergrowth. SSZ-26 zeolite is an alumino-silicate constituted by an intergrowth.
The silica-alumina composition of the three structures appears to be similar. However, their preparation (that of SSZ-26 and CIT-1) requires expensive structuring agents (templates) and lengthy synthesis processes. Thus SSZ-26 zeolite has been described in U.S. Pat. No. 4,910,006 and U.S. Pat. No. 4,963,337 describes SSZ-33 borosilicate, in its aluminium form, hereby incorporated by reference.
Such zeolites are synthesised in the presence of alkaline ions supplied by sodium hydroxide. These ions must be eliminated by at least one ion exchange step, for example using ammonium cations, and a calcining step, which complicates the preparation process and renders it more expensive.